SM22α inhibits lamellipodium formation and migration via Ras-Arp2/3 signaling in synthetic VSMCs.

We previously demonstrated that smooth muscle (SM) 22α promotes the migration activity in contractile vascular smooth muscle cells (VSMCs). Based on the varied functions exhibited by SM22α in different VSMC phenotypes, we investigated the effect of SM22α on VSMC migration under pathological conditions. The results demonstrated that SM22α overexpression in synthetic VSMCs inhibited platelet-derived growth factor (PDGF)-BB-induced cell lamellipodium formation and migration, which was different from its action in contractile cells. The results indicated two distinct mechanisms underlying inhibition of lamellipodium formation by SM22α, increased actin dynamic stability and decreased Ras activity via interference with interactions between Ras and guanine nucleotide exchange factor. The former inhibited actin cytoskeleton rearrangement in the cell cortex, while the latter significantly disrupted actin nucleation activation of the Arp2/3 complex. Baicalin, a herb-derived flavonoid compound, inhibited VSMC migration via upregulation of SM22α expression in vitro and in vivo. These data suggest that SM22α regulates lamellipodium formation and cell migration in a phenotype-dependent manner in VSMCs, which may be a new therapeutic target for vascular lesion formation.

Phosphorylation of smooth muscle 22α facilitates angiotensin II-induced ROS production via activation of the PKCδ-P47phox axis through release of PKCδ and actin dynamics and is associated with hypertrophy and hyperplasia of vascular smooth muscle cells in vitro and in vivo.

RATIONALE: We have demonstrated that smooth muscle (SM) 22α inhibits cell proliferation via blocking Ras-ERK1/2 signaling in vascular smooth muscle cells (VSMCs) and in injured arteries. The recent study indicates that SM22α disruption can independently promote arterial inflammation through activation of reactive oxygen species (ROS)-mediated NF-κB pathways. However, the mechanisms by which SM22α controls ROS production have not been characterized. OBJECTIVE: To investigate how SM22α disruption promotes ROS production and to characterize the underlying mechanisms. METHODS AND RESULTS: ROS level was measured by dihydroethidium staining for superoxide and TBA assay for malondialdehyde, respectively. We showed that downregulation and phosphorylation of SM22α were associated with angiotensin (Ang) II-induced increase in ROS production in VSMCs of rats and human. Ang II induced the phosphorylation of SM22α at Serine 181 in an Ang II type 1 receptor-PKCδ pathway-dependent manner. Phosphorylated SM22α activated the protein kinase C (PKC)δ-p47phox axis via 2 distinct pathways: (1) disassociation of PKCδ from SM22α, and in turn binding to p47phox, in the early stage of Ang II stimulation; and (2) acceleration of SM22α degradation through ubiquitin-proteasome, enhancing PKCδ membrane translocation via induction of actin cytoskeletal dynamics in later oxidative stress. Inhibition of SM22α phosphorylation abolished the Ang II-activated PKCδ-p47phox axis and inhibited the hypertrophy and hyperplasia of VSMCs in vitro and in vivo, accompanied with reduction of ROS generation. CONCLUSIONS: These findings indicate that the disruption of SM22α plays pivotal roles in vascular oxidative stress. PKCδ-mediated SM22α phosphorylation is a novel link between actin cytoskeletal remodeling and oxidative stress and may be a potential target for the development of new therapeutics for cardiovascular diseases.